The present invention relates to organic-inorganic, hybrid silicates and metal-silicates having an ordered structure and a process for the preparation thereof.
Silicates and metal-silicates are a group of compounds which can produce three-dimensional compact or porous (zeolites), lamellar (micas and clays) or linear crystalline structures. Zeolites and clays have been extremely important in the evolution of catalytic processes and for the separation of blends of different molecules. Their properties are correlated with the geometry of the crystalline structure and chemical composition, which determine the acidic and polar characteristics. Zeolites, in particular, are crystalline-porous solids having a structure consisting of a three-dimensional lattice of TO4 tetrahedra which are connected by means of oxygen atoms, wherein T is a tri- or tetravalent tetrahedral atom, for example Si or Al.
The substitution of Si or Al with other elements, such as Ge, Ti, P, B, Ga and Fe, for example, has allowed the modification of the physical-chemical properties of materials, obtaining products with new properties, used as catalysts or molecular sieves.
Studies aimed at even more deeply modifying the properties of these materials are underway, for synthesizing organic-inorganic hybrids, in which at least a part of the silica precursor consists of mixed silicates containing at least one Si—C bond. In particular, attempts have been made at synthesizing structures of crystalline-porous silicates or metal-silicates containing organic groups inside the lattice, starting from disilane precursors in which an organic group is linked to two silicon atoms.
In Nature 416, 304-307 (Mar. 21, 2002) Inagaki et al. describes the synthesis of an ordered hybrid, mesoporous silicate containing ≡Si—C6H4—Si≡ groups. This material has a hexagonal pore distribution with a constant lattice of 52.5 Å and walls which delimit the pores with a structural periodicity equal to 7.6 Å along the direction of the channels. The material was synthesized by adding 1,4-bis(triethoxysilyl)benzene to an aqueous solution containing octadecyltrimethylammonium chloride, as surfactant, and soda. The X-ray powder diffraction pattern shows 3 reflections at low angular values (2θ<4.0°), with 2θ=1.94°, 3.40°, 3.48°, corresponding to distances between planes d=45.5, 26.0, 22.9 Å and 4 reflections in the region 10°<2θ<50° (2θ=11.64°, 23.40°, 35.92°, 47.87° corresponding to d=7.6, 3.8, 2.5 and 1.9 Å) A further reflection was localized at about 20.5° of 2θ, but it was large and badly defined.
JP2002-211917-A describes the introduction of at least one ≡Si—R≡Si≡ unit in the structure of known zeolite phases. In particular MFI, LTA, MOR structures are described, wherein a small amount of the oxygen as the bridge between two silicon atoms (≡Si—O—Si≡) is substituted by methylene groups (≡Si—CH2—Si≡). Examples are provided of ratios of silicon bound to the carbon, with respect to the total silicon T/(Q+T) not higher than 10%. In this ratio, heteroatoms other than silicon possibly present in the structure, such as aluminium, are not considered.
The syntheses are carried out using bis-triethoxysilyl-methane (BTESM) as silica source, possibly in the presence of tetraethylorthosilicate. The synthesis method used is that used for the synthesis of known zeolite structures and templates are possibly used. Important breakage phenomena of the Si—C bonds are always observed under the synthesis conditions described, therefore only an aliquot of the above bond remains integral in the final structure.
According with this, the 29Si-MAS-NMR spectra of the samples show a minor signal at −60 ppm, attributed to the presence of Si—C bonds. Furthermore, intense signals are present, also in samples prepared using BTESM only as silica source, attributed to Q4 sites (about −115 ppm) and Q3 (about −105 ppm), corresponding to Si atoms surrounded by four tetrahedra O—SiO3 and three tetrahedra O—SiO3 and an —OH group, respectively. This confirms a considerable breakage of the Si—C bond of the precursor BTESM.
Materials having very different properties with respect to their inorganic correspondents have therefore not been obtained, probably due to the low substitution level of the ≡Si—O—Si≡ groups with ≡Si—CH2Si≡ groups.
New hybrid, organic-inorganic silicates and metal-silicates having an ordered structure have now been found, useful, for example, in the field of catalysis, in the separation of compounds in blends and nanotechnologies.